But it is not sufficient that these terms be understood in practice; the critical art requires, that their meaning be traced to its foundation in human nature. The relations that connect objects together, have been examined in more than one view. Their influence in directing the train of our perceptions, is handled in the first chapter; and in the second, their influence in generating passion. Here they must be handled in a new view; for they are clearly the occasion of congruity and propriety. We are so framed by nature, as to require a certain suitableness or correspondence among things connected by any relation. This suitableness or correspondence is termed _congruity_ or _propriety_; and the want of it, _incongruity_ or _impropriety_. Among the many principles that compose the nature of man, a sense of congruity or propriety is one. Destitute of this sense, we could have no notion of congruity or propriety: the terms to us would be unintelligible[2]. As this sense is displayed upon relations, it is reasonable beforehand to expect, that we should be so formed, as to require among connected objects a degree of congruity proportioned to the degree of the relation. And upon examination we find this to hold in fact. Where the relation is strong and intimate as betwixt a cause and its effect, a body and its members, we require that the things be suited to each other in the strictest manner. On the other hand, where the relation is slight, or accidental, as among things jumbled together in the same place, we demand little or no congruity. The strictest propriety is required in behaviour and manner of living; because a man is connected with these by the relation of cause and effect. The situation of a great house ought to be lofty; for the relation betwixt an edifice and the ground it stands upon, is of the most intimate kind. Its relation to neighbouring hills, rivers, plains, being that of propinquity only, demands but a small share of congruity. Among members of the same club, the congruity ought to be considerable, as well as among things placed for show in the same niche. Among passengers in a stage-coach, we require very little congruity; and less still at a public spectacle. Congruity is so nearly allied to beauty, as commonly to be held a species of it. And yet they differ so essentially, as never to coincide. Beauty, like colour, is placed upon a single subject; congruity upon a plurality. Further, a thing beautiful in itself, may, with relation to other things, produce the strongest sense of incongruity. Congruity and propriety are commonly reckoned synonymous terms; and hitherto in opening the subject they are used indifferently. But they are distinguishable; and the precise meaning of each must be ascertained. Congruity is the genus, of which propriety is a species. For we call nothing propriety, but that congruity or suitableness which ought to subsist betwixt sensible beings and their thoughts, words, and actions.